An image sensor is a semiconductor device that converts an optical image into an electrical signal, and is generally classified as a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
In the related art, a photodiode is formed on a substrate using an ion implantation method. However, as the size of the photodiode is gradually reduced in order to increase the number of pixels without increasing chip size, the area of a light receiving part is reduced, which tends to reduce image quality.
Moreover, although there exists a method of increasing electron generation rate by increasing capacitance of the photodiode, there is a limitation in expanding a depletion region of the photodiode in order to increase capacitance. In addition, a photo aperture ratio is lowered by a structure formed by a back end process of the photodiode.
As one of alternative proposals for solving the problem as described above, the photodiode is formed with amorphous silicon (Si) or a structure where a readout circuitry is formed on a silicon (Si) substrate (main substrate) and the photodiode is formed on another substrate (donor substrate) and bonded to the upper portion of the readout circuitry through a wafer-to-wafer bonding method is attempted (for reference, referred to as “three-dimensional image sensor” and “PD-up CIS”).
The structure as described above is made by forming a p+ region, an n− region, and an n+ region in the photodiode region of the donor substrate in sequence and bonding the donor substrate to the main substrate.
However, when a pixel trench isolation (PTI) process is performed for isolating between the photodiodes forming a pixel unit after the donor substrate is coupled to the main substrate, excessive plasma damage due to the etching is generated, causing a problem that a plurality of dark defects are generated at a chip edge.
This causes the low illumination characteristics of the image sensor to degrade and the production yield to reduce.